Conveyor belts generally comprise a foil composed of a fusible thermoplastic or of thermoplastic elastomers, so that the two ends of the conveyor belt can be bonded by way of a combined fusion and welding process, to form a continuous conveyor belt. At the same time, the fusible foil forms the belt surface on which the product to be transported is conveyed. In order to modify the belt surface as a function of the required use, e.g. to render the belt surface adhesive or less adhesive, or to increase its scratch resistance or its chemicals resistance, new development work has always hitherto had to be carried on the thermoplastic foil.
In order to eliminate this new development work, surface-modifying outer layers (e.g. composed of Teflon) have in some instances previously been laminated or calendared onto conveyor belts. The problem with this surface modification of conveyor belts is that release of the coating has to be avoided even during long-term operation of the conveyor belt and during repeated flexing over the deflector rolls. This problem is amplified by the fact that the conveyor belt itself has to have some degree of elasticity because during flexing over the deflector rolls the outer side of the conveyor belt is subjected to scratching and bending. The modifying outer layer has to accept involvement in these stretching and bending processes.
The coating of rigid articles, such as plastics bottles or plastics pipes, is sometimes carried out by the plasma-coating process in radio-frequency plasma. However, the problem of adequate adhesion of the coating during flexing, bending or stretching of the substrate does not arise here.
The radio-frequency plasma coating of packaging foils is likewise known. Although packaging foils are flexible they are not very elastic (the modulus of elasticity to DIN 53 455 of plastics used for packaging foils, e.g. polypropylene, being typically markedly greater than 1000 N/mm2) and the amount of surface elongation likely to occur during flexing is therefore only very small. Again, the flexing of the foil here takes place only once, namely during packaging of the product. The requirements placed upon the adhesion of a plasma coating here are therefore not comparable with the requirements for the adhesion of outer layers in conveyor belts.
One known problem in the plasma coating of plastics is the tendency of the plastics substrate to evolve gases in the vacuum in which the plasma-coating process is undertaken, and possibly also to release residual contents of volatile monomers. These gases accumulate under the (low-gas-permeability) plasma-polymer layer as it forms, and can weaken its adhesion to the plastics substrate. This effect becomes amplified as the plasma-coating time increases, because over the course of time the outer layer formed becomes increasingly thick and increasingly impermeable to gases. The evolution of gases can be eliminated only to some extent via prior storage of the plastic in a vacuum for a prolonged period.